Crude petroleum oil charged to a petroleum refinery contains a number of impurities harmful to the efficient operation of the refinery and detrimental to the quality of the final petroleum product.
Oil insoluble mineral salts, such as the chlorides, sulfates and nitrates of sodium, potassium, magnesium, calcium, and iron are present, generally in the range of 3 to 200 pounds per thousand barrels (ptb) of crude (calculated, by convention, as NaCl). The mineral salts of the less alkaline metals, such as magnesium, calcium, and iron, are acidic. Oil insoluble solids, such as the oxides and sulfides of iron, aluminum, and silicon are also present. Oil soluble or colloidal metal soaps of sodium, potassium, magnesium, calcium, aluminum, copper, iron, nickel, and zinc, and oil soluble organometallic chelants, such as porphyrins of nickel and vanadium, may be found in various concentrations. These metal species contribute to corrosion, heat exchanger fouling, furnace coking, catalyst poisoning, and end product degradation and devaluation.
In addition, oil soluble or colloidal acidic species, such as the hydrochloride salts of sufficiently hydrocarbonaceous basic nitrogen compounds (e.g., amines), organic sulfoxy, phenolic, and carboxylic acids, such as naphthenic acids (C.sub.n H.sub.2n O.sub.2), are present to varying degrees in petroleum crude. These acids also contribute to various corrosion problems.
The primary corrodent of the main fractionator unit atmospheric distillation tower overhead and other regions of the refinery system where temperatures are elevated and water condenses, is hydrochloric acid (HCl). This gas is produced at the high temperatures in the bottom of the distillation tower, primarily via three reactions:
1. Hydrolysis of Mg Cl.sub.2.2H.sub.2 O and CaCl.sub.2.2H.sub.2 O PA1 2. Metathesis of NaCl and organic acids PA1 3. Pyrolysis of amine hydrochloride salts
The evolution of HCl is reduced primarily by washing the water soluble precursors, such as MgCl.sub.2, CaCl.sub.2, NaCl and the smaller, more hydrophilic organic acids and amines, including ammonia, from the raw crude oil in a single or multi-stage desalter. Other halide salts such as those of bromide and fluoride which have been found to also cause corrosion can also be reduced in this manner.
Crude oil desalting is a common crude oil purification method where an emulsion is formed by adding water in the amount of approximately 2.5% to 10% by volume of the crude oil at temperatures from about 150.degree. F. to 300.degree. F. The added water is intimately mixed into the crude oil to contact the impurities therein in order to transfer these impurities into the water phase of the emulsion. The emulsion's intimacy and subsequent resolution is usually effected with the assistance of emulsion making and breaking surfactants, and by the known method of providing an electrical field to polarize the water droplets. As the emulsion is broken, the water phase and petroleum phase are separated and subsequently removed from the desalter vessel. The petroleum phase is next directed to the distillation train where it is fractionated for further processing downstream. The effluent brine, the pH of which is kept between 5 and 9, typically 6 and 8, is sent to the wastewater treatment unit.
Some of the impurities attempted to be removed by this method remain with the petroleum and ultimately result in the corrosion and fouling problems previously described. Various concepts which have attempted to resolve these continuing problems are described hereinbelow.